Tyrosine Hydroxylase plays an important role in the physiology of adrenergic neurons. It is the first and rate-limiting enzyme involved in the biosynthesis of the catecholamines Dopamine and Norepinephrine from tyrosine. TH is, therefore, a useful marker for dopaminergic and noradrenergic neurons. The enzymatic activity of TH requires ferrous ions as cofactors and is believed to be regulated by phosphorylation. At least four isoforms of human TH have been identified which result from alternative splicing.